It's certainly difficult to weigh up various considerations such as peace, justice and historical interest.
Will you please not give such
reasonable reactions?

But more below.
For the most part, the Northern Ireland peace process has been going ahead on the basis of putting peace before justice, with certain amnesties being granted. I don't think that has been extended to civilian murders though.
The word "civilian" raises another interesting point: what counts as "civilian" here? As far as I can read, the IRA at the time thought that Jean McConville was an informant for the Unionist side.
And just to be clear: I don't want in any way, shape or form downplay the vileness and the gruesomeness of the murder, nor how utterly wrong they were in their assumptions about Ms. McConville.
In this case, it is alleged that a separate IRA member implicated Adams, and the evidence for this case did not come from the interviews.
It is a difficult dilemma when it comes to information about cases of murder and weighing up justice and historical interest, I think.
Then history will have to suffer, the idea that people can confess to horrendous crimes without being held to account is against natural justice. In most countries there is the equivalent of Queen's evidence for when the admission of a single persons' crimes is required for the purpose of justice.
I had to look up what "Queen's evidence" is and then found out the Dutch term translates to "Crown witness"

. No contest from me here; I don't dispute that the public prosecutor should have the possibility to coax witnesses, typically small criminal fish, into singing against their former criminal associates in exchange for a lessened sentence or even immunity.
But that's not what's happened here. Two former IRA members, Brendan Hughes and Dolores Price, have made claims to Adams' involvement in the murder, both in 2010. Hughes was already dead - the statement was in a book published posthumously - but Price was alive and kicking. Subsequently, the Northern Irish Police has requested the taped interviews to be released. As far as I can see from various newspaper articles about it, the request was not limited to the interviews of Hughes and Price. And in reaction to this, several other interviewees of the project have requested their interviews to be destroyed. They have all been interviewed on the assurance that those interviews would not be released during their own lifetime.
At least in the case of Price, that was not the case, and I don't see why the police couldn't just interview Price themselves to retell her tale about the McConville murder, possibly with a promise of a lesser sentence or immunity for herself.
There are long-standing cases of confidentiality: doctors, attorneys, journalists, priests all have privilege about their communication with their clients and sources. And there are more.
Suppose a former IRA member would leave a sealed envelope with their attorney, with the instruction it only be opened after their death, and the police got wind of it. Would you approve the police go after the attorney to seize the envelope?
Or would you approve the police go after the priest of a former IRA member and grill him about what was said during confession?
Another example of privilege is the
Dutch Safety Board, which investigates accidents. By law, its reports cannot be entered into evidence in criminal proceedings, and it will only release testimonies to the prosecutor on the express consent of the witness. The goal of the safety board is to learn lessons from accidents so similar accidents won't happen again. That goal could not be served if parties could not freely speak to the board.
Just as an attorney can't do his job properly without the client-attorney privilege, and a journalist can't do his job without promising confidentiality to (some of) his sources, a contemporary historian can't do it either with promising confidentiality in subjects like this. The Boston Tapes simply would not exist if the interviewees had not had the expectation of confidentiality of their statements. Without that promise, the people involved would keep their mouths shut and take their secrets with them to their graves.
And in fact, the family of other such murder cases is also better served by confidentiality, because it means that in 10, 20 years time the circumstances around the murder may very well be revealed, while without confidentiality, it will always remain unclear.